Choose right variety to lower the risk

GROWERS can reduce the risk by choosing varieties with as high a disease resistance rating as possible for the dominant expected diseases.

"Resistant varieties are the first line of defence," says Dr Hardwick. "That is because they carry less disease and so a fungicide has to work less hard to keep infection under control."

It also helps when bad weather prevents fungicides being sprayed on time, he says. "It means there will at least be some protection from disease already in place."

Despite these in-built defences fungicides will be the mainstay of disease control for the foreseeable future. "So it is vital that they are protected from the threat of resistance."

There has been a gradual increase in the occurrence of fungicide resistance since systemic types arrived in the early 1970s. These products often have very specific modes of action, unlike many of the older materials.

Fungicide resistance can arise rapidly and completely, so that disease control is totally lost. It can also occur more gradually resulting in partial loss of control. Both types are known in the UK.

"There are many instances of complete failure of control due to resistance in the MBC fungicides, for example benomyl and carbendazim for eyespot control, and some to the phenylamide group such as metalaxyl for control of potato late blight," warns Dr Hardwick.

"More recently there have been cases of such resistance to the newest group, the strobilurins," he says.

"The recent detection in Germany of reduced sensitivity in the powdery mildew pathogen to quinoxyfen, one of the main ingredients for its control, highlights the importance of adopting at anti-resistance strategy at the outset."

A more gradual loss of control has been found with the triazole group.

Resistance to some groups of fungicides, for example MBCs, has occurred more often than to others. That is because the active ingredients affect only one biochemical process within the pathogen and are more easily bypassed than those which operate on several processes, he says.

"Likewise some pathogens appear to be more likely than others to become resistant." These, which include the fungi causing powdery mildew and potato late blight, produce several generations a season and so are subjected to greater selection pressure.

Factors which affect the development resistance include the type of fungicide, its frequency of use, whether alone or in a programme, the target pathogen and the robustness or ability of resistant forms to survive. &#42